Various types of thermal duplicating processes, that is, processes which involve the thermal transfer of heat-sensitive material, are well known in the art. See, for example, those processes described in U.S. Pat. Nos. 3,122,997 and 3,122,998 issued to Raczynski et al., as well as U.S. Pat. Nos. 1,514,677, 2,501,445, 2,611,313, 2,808,777, 2,939,099, 3,809,748, 3,181,965, 3,260,603, 3,262,386, 3,267,848, 3,283,708 3,293,055, 3,304,015, 3,384,015 and our U.S. Pat. No. 3,706,276. As further background hereto, reference may also be made to U.S. Pat. Nos. 2,191,514, 2,398,779, 2,408,147, Re. 24,899, 3,010,390, 3,458,336 and 3,476,937.
Typically, an original to be duplicated is placed in an assembly with a transfer sheet substrate carrying a fusible layer of heat sensitive material and an image-receiving sheet which has its receiving surface in contact with the layer of fusible material. The position and facing of the original document sheet relative to the transfer and receiving sheets can be varied depending upon whether a right-reading or a laterally reverse reading image is desired on the image receiving sheet. The image on the original document must be defined by an infrared absorptive material such as carbon, heavy metal, or certain organic compounds so that the image areas of the original document, upon exposure, will absorb more infrared radiation than the surrounding, non-image areas. The absorbed infrared radiation is converted to thermal energy forming a thermal pattern in the original document which corresponds to the visible image pattern. This heat pattern is conducted through the substrate of the transfer sheet to the heat sensitive layer. The heat sensitive layer is selectively fused in correspondence to the image and the fused material is transferred to the image-receiving sheet.
Following image transfer, depending upon the manner of placement of the original document, the image-receiving sheet may serve as a facsimile copy of the original document or it may be utilized as a master in a solvent duplicating or lithographic printing process. The heat-sensitive layer of the transfer sheet should contain, in addition to the fusible material, the necessary components for the ultimate application of the transferred image. For example, in spirit duplication processes, the heat-sensitive layer contains wax, or other fusible substance, mixed with an alcohol soluble dye (the combination being known to the art as "carbon") to produce the image color in the ultimate copy. The waxy material is thermally transferred in reverse reading fashion to the image-receiving sheet, normally referred to as a master. The master is placed on the drum of a duplicating machine and contacted with a succession of sheets of copy paper previously wet with a volatile alcohol solvent for the dye. The solvent dissolves part of the dye in the master image and transfers it to the copy paper. In another type of solvent duplicating process, the transferred image material contains a chemical reagent which reacts with a second reagent on the copy paper to yield a color. The second reagent may be originally in the copy paper or it may be delivered to the paper in duplicator fluid applied prior to contact with the master sheet. In offset lithographic printing, the material transferred to the image-receiving sheet, normally referred to as a printing plate, defines a right-reading image. If a reverse-reading image is initially prepared on an image-receiving sheet, it can be transferred to a direct image lithographic plate by contacting the thermally produced image with the face of the lithoplate and heating, as known to the art.
Thermal image transfer processes are relatively inexpensive, and effective in terms of color density when reproducing high contrast originals. However, such processes are generally not suitable for reproducing continuous tones as are found in silver photographic prints. When such an original is used for thermal reproduction with presently available carbon transfer processes, high contrast copies are produced which lack intermediate tones.
The present invention enables the production of continuous tone thermal copies. In particular, we provide a thermal transfer sheet having a discontinuous transfer layer defined by a plurality of closely spaced but discrete carbon regions which serve to "break" the image up into a plurality of image regions. In a specific embodiment, there is a substantially uniform distribution of carbon regions of varying diameters which provide continuous tone reproduction through temperature-correlated carbon transfer, as referred to in more detail hereinafter.